There is a continuing need to improve the heat resistance and air retention of pneumatic tires. Elastomeric materials that do so can increase the safety and performance of vehicles by maintaining the proper tire inflation for longer periods, and they can also increase fuel economy by reducing weight. A continuing problem, however, in the tire and innerliner industry is the ability to improve the processability of air barriers, such as innerliners, without compromising air retention and durability of the tire itself.
Bromobutyl and chlorobutyl rubbers are typically used for air-retention in tubeless tires. Brominated poly(isobutylene-co-p-methylstyrene) (BIMS or BIMSM), such as those disclosed in U.S. Pat. Nos. 5,162,445 and 5,698,640, have been used when heat resistance, inter alia, is of importance. The selection of ingredients for the commercial formulations of elastomers depends upon the balance of properties desired and the application and end use. For example, in the tire industry, processing properties of the green (uncured) compound in the tire plant versus in-service performance of the cured rubber tire composite, and the nature of the tire, i.e. bias versus radial tire, and passenger versus truck versus aircraft tire are all important considerations that must be balanced.
One method to alter product properties and improve air barrier properties has been to add clays (such as nanoclays or organoclays) to elastomers to form a “nanocomposite.” Nanocomposites are polymer systems containing inorganic particles with at least one dimension in the nanometer range (see for example WO 2008/042025). Common types of inorganic particles used in nanocomposites are phyllosilicates, an inorganic substance from the general class of so called “nano-clays” or “clays” generally provided in an intercalated form wherein platelets or leaves of the clay are arranged in a stack in the individual clay articles with interleaf spacing usually maintained by the insertion of another compound or chemical species between the adjacent lamellae. Ideally, intercalation should take place in the nanocomposite, wherein the polymer inserts into the space or gallery between the clay surfaces. Ultimately, it is desirable to have exfoliation, wherein the polymer is fully dispersed with the individual nanometer-size clay platelets. Due to the general enhancement in air barrier qualities of various polymer blends when clays are present, there is a desire for a nanocomposite with low air permeability; especially a vulcanized elastomer nanocomposite such as used in the manufacture of tires.
The extents of dispersion, exfoliation, and orientation of platy nano-fillers such as organosilicates, mica, hydrotalcite, graphitic carbon, etc., could strongly influence the permeability of the resulting polymer nanocomposites. The barrier property of a polymer in theory is significantly improved, by an order of magnitude, with the dispersion of just a few volume percent of exfoliated high aspect-ratio platy fillers, due simply to the increased diffusion path lengths resulting from long detours around the platelets. Nielsen, J. Macromol. Sci. (Chem.), vol. A1, p. 929 (1967), discloses a simple model to determine the reduction in permeability in a polymer by accounting for the increase in tortuousity from impenetrable, planarly oriented platy fillers. Gusev et al., Adv. Mater., vol. 13, p. 1641 (2001), discloses a simple stretched exponential function relating the reduction of permeability to aspect ratio times volume fraction of the platy filler that correlates well with permeability values numerically simulated by direct three-dimensional finite element permeability calculations.
For rubber compounding applications, small sub-micron fillers such as carbon black and silica are used for fatigue resistance, fracture toughness and tensile strength. Filler particles larger than a micron act tend to concentrate stress and initiate defects. Thus, platy nanofillers added to reduce permeability are desirable in elastomers. To maximize the effect of aspect ratio on permeability reduction, it is useful to maximize the degree of exfoliation and dispersion of the platelets, which are generally supplied in the form of an intercalated stack of the platelets. However, in isobutylene polymers, dispersion and exfoliation of platy nanofillers requires sufficient favorable enthalpic contributions to overcome entropic penalties. As a practical matter, it has thus proven to be very difficult to disperse ionic nanofillers such as clay into generally inert, nonpolar, hydrocarbon elastomers. The prior art has, with limited success, attempted to improve dispersion by modification of the clay particles, by modification of the rubbery polymers, by the use of dispersion aids, and by the use of various blending processes.
The “inertness” of saturated hydrocarbon polymers such as BIMSM, their low reactivity and incompatibility with most other materials, and the difficulties in adhering them to, or using them in conjunction with most other materials has restricted their use in many areas. Chemical modification of the elastomers, modification of the blend component, and the use of additional compatibilizing blend components, has been attempted. U.S. Pat. No. 5,162,445 discloses a method to improve polymer blend compatibility or blend co-curability by copolymerizing an unsaturated comonomer and/or a comonomer having reactive functionality with isobutylene. U.S. Pat. No. 5,548,029 discloses graft copolymers of isobutylene-p-methylstyrene copolymers to compatibilize blends of saturated and unsaturated elastomers.
U.S. 2006/0229404 discloses a method for making compositions of an elastomer with exfoliated graphite in which the diene monomers are polymerized in the presence of 10 phr or more exfoliated graphite so that the graphite is intercalated with the elastomer.
U.S. Pat. No. 6,548,585 discloses refrigerant hoses made with an inner tube composition of a brominated copolymer rubber such as BIMSM with an inorganic lamellar compound such as graphite, zirconium phosphate, calcogenides, talc, kaolinite, benotnite, montmorillonite, mica, chlorite, etc.
The presence of such nanoscale materials has a tendency to increase viscosity and so make the processability of the elastomeric compositions more difficult. Processing aids such as naphthenic, paraffinic, and aliphatic resins can be added to the elastomeric compositions to combat such issues. See, for example, U.S. Pat. No. 4,279,284. However, the improved processability due to the presence of oils and resins may result in a loss of air impermeability and undesirable color, among other undesirable effects of various other properties.
The preparation of BIMSM-clay nanocomposites from melt-blending, solution blending and an emulsion process are disclosed in commonly assigned U.S. application Ser. No. 11/183,361, Split-Stream Process for Making Nanocomposites, by W. Weng et al., filed Jul. 18, 2005; and commonly assigned U.S. application Ser. No. 11/184,000, Functionalized Isobutylene Polymer-Inorganic Clay Nanocomposites and Organic-Aqueous Emulsion Process, by W. Weng et al., filed Jul. 18, 2005 (published as U.S. Patent Publication No. 2007-0015853, Jan. 18, 2007).
There is still a need, therefore, for improving the processability of elastomeric compositions useful for tires, air barriers, among other things requiring air retention, while maintaining or improving the air impermeability of those compositions.